Print correct startup cost for the group aggregate.

Hi,

While reading through the cost_agg() I found that startup cost for the
group aggregate is not correctly assigned. Due to this explain plan is
not printing the correct startup cost.

Without patch:

postgres=# explain select aid, sum(abalance) from pgbench_accounts where
filler like '%foo%' group by aid;
QUERY
PLAN
-------------------------------------------------------------------------------------
GroupAggregate (cost=81634.33..85102.04 rows=198155 width=12)
Group Key: aid
-> Sort (cost=81634.33..82129.72 rows=198155 width=8)
Sort Key: aid
-> Seq Scan on pgbench_accounts (cost=0.00..61487.89 rows=198155
width=8)
Filter: (filler ~~ '%foo%'::text)
(6 rows)

With patch:

postgres=# explain select aid, sum(abalance) from pgbench_accounts where
filler like '%foo%' group by aid;
QUERY
PLAN
-------------------------------------------------------------------------------------
GroupAggregate (cost=82129.72..85102.04 rows=198155 width=12)
Group Key: aid
-> Sort (cost=81634.33..82129.72 rows=198155 width=8)
Sort Key: aid
-> Seq Scan on pgbench_accounts (cost=0.00..61487.89 rows=198155
width=8)
Filter: (filler ~~ '%foo%'::text)
(6 rows)

PFA patch to correct the same.

Regards,
Rushabh Lathia
www.EnterpriseDB.com